Title (2/5)

I would have preferred if the sequence of the letters followed that of the development of their relationship. From strangers to friends to best friends and eventually lovers. 

Letter 1 was fine, but you talked about the acceptance of her parents of their close relationship before it was clear they were becoming close enough to have each other over at their homes. Also, it seems they were already close before falling in love that night but his caller ID still showed "stupid miyeon" when it was obvious he cared for her.

Letter 2: He should have addressed the front part pertaining to their relationship and not just the insane girl. 

Letter 4: How did it go from comforting her to her huge appetite? The current timeline and flashback does not tally.

Letter 5: The first letter mentioned they were partners for four years and it seems that they never bothered getting close to each other in the four years because she haven't seen any good side of him but here it states she fell in love with him after four years with a mention of another additional year. This is confusing because I am unable to tell when the feelings started.

I understand that their one year anniversary falls on the 22nd March. But she died one week before so the letter has to be dated earlier? Also you said Jongin re-read the letter, meaning he had already read it once before the actual date he was supposed to receive the letter. I can understand that she wrote the letter before she got into the accident but it could be nicer if this was the first time he was reading it.

As beautiful as the ending was, it is confusing because as mentioned above, he read it once when she was still around, which is not on their one year anniversary and he reads it again whilst celebrating their one year anniversary alone. You could have even made this their second anniversary or even longer, to prove that his strong love for her never faded after losing her during their one year anniversary? It varies and this are simply suggestions.